There is an increasing need for lower fuel consumption in automobiles and for tires with smaller rolling resistance. Thus, it is desirable to provide a rubber composition that has a low tan δ (hereinafter, referred to as a “low loss factor”) and excellent low exothermic property as a rubber composition for use in tire treads, and so on. In addition, a rubber composition for treads is also required to have excellent wear resistance and fracture resistance in addition to the low loss factor. To this extent, it is effective to improve the affinity between a filler such as carbon black or silica and rubber components in a rubber composition for improved low loss factor, wear resistance and fracture resistance of the rubber composition.
For example, to improve the affinity between a filler and rubber components in a rubber composition and to enhance the reinforcing effect by the filler, a synthetic rubber with improved affinity for a filler by chain end modification, a synthetic rubber with improved affinity with a filler through copolymerization of functional group-containing monomers, and so on have been developed.
On the other hand, while natural rubber has been adopted abundantly making use of its superior physical properties, no technique has been established to improve natural rubber itself so that it has a greater affinity for a filler and to significantly enhance the reinforcing effect by the filler.
For example, some techniques have been proposed to epoxidize natural rubber, which, however, does not sufficiently improve the affinity between the natural rubber and a filler, and therefore fails to enhance the reinforcing effect by a filler adequately. In addition, other techniques are known to add a vinyl-based monomer to natural rubber latex for graft polymerization (see PTL 1-3 below), and the grafted natural rubber obtained by these techniques has already been produced as MG latex and the like. However, high purity latex is required to achieve high graft efficiency by these techniques. Accordingly, there is a problem that necessitates separate operation such as centrifugal separation, resulting in an increase in manufacturing costs.
As such, for the purposes of improving the reinforcing performance of and the affinity for a filler and reducing manufacturing costs, JP 2004-262973 A (PTL 4) discloses a rubber composition that is formed blending a modified natural rubber, which results from graft polymerization and subsequent coagulation and drying of a polar group-containing monomer onto natural rubber latex, with carbon black or silica. In addition, as a technique for reducing manufacturing cost, JP 2006-152171 A (PTL 5) discloses a modified natural rubber that is formed through graft polymerization or attachment of a polar group-containing compound to at least one natural rubber raw material selected from the group consisting of natural rubber under mechanical shear force, a natural rubber latex coagulation and a natural rubber cup lump.
The techniques disclosed in PTL 4 and PTL 5 allow production of a modified natural rubber at low cost that is capable of improving the low loss factor, wear resistance and fracture resistance of the rubber composition. However, the techniques of PTL 4 and PTL 5 need further improvement in terms of improvement of the efficiency of graft polymerization and attachment reaction between the natural rubber raw material and the polar group-containing compound.